The effect of quenching temperature on the hardness and grain size of 22Cr2Ni4MoV steel is a critical factor in determining the mechanical properties of this high-hardenability low-carbon alloy steel. In this study, diesel was used as the quenching cooling medium. It was observed that both hardness and grain size are significantly influenced by the quenching temperature. The highest values were achieved at a quenching temperature range of 850–900°C, where the hardness reached up to 47 HRC, and the grain size was approximately 8–15. However, as the quenching temperature increased beyond this range, both hardness and grain size began to decrease sharply. This decline suggests that the optimal quenching temperature for this material is within the 850–900°C range. Subsequent tests were conducted using 850°C quenching with diesel cooling to further evaluate the microstructure and properties.
The impact of tempering temperature on hardness and impact toughness of Cr2Ni4MoV steel was also analyzed. During the low-temperature tempering phase (below 250°C), the hardness and impact toughness decreased gradually. However, once the tempering temperature exceeded 250°C, the hardness dropped more rapidly with increasing temperature. At the same time, the impact toughness decreased significantly, reaching its minimum at around 480°C, where the impact energy was recorded at 35 J. After that, as the tempering temperature rose above 500°C, the impact toughness started to increase linearly. A temper embrittlement zone was identified between 300–550°C, which can affect the material's ductility and toughness.
In addition, the effect of tempering temperature on the strength and plasticity of 22Cr2Ni4MoV steel was studied. The tensile strength and yield point showed a linear decrease as the tempering temperature increased. Meanwhile, the elongation and reduction in area exhibited a slight dip in the 250–500°C range, aligning with the brittle zone observed earlier. This indicates that the material becomes more ductile after higher-temperature tempering.
Microstructural analysis revealed that 22Cr2Ni4MoV steel exhibits good hardenability. When air-cooled after forging, it forms a martensite structure (a). After normalization at 920°C followed by tempering at 680°C, the microstructure transformed into ferrite + pearlite (b). Oil quenching at 850°C resulted in a martensite + trace residual austenite structure (c). Tempering at 200°C after 850°C quenching produced a tempered martensite + small amount of residual austenite (d). Further tempering at 480°C led to a tempered sorbite structure (e). When quenched at 975°C, a martensite structure with distinct grain boundaries was observed (f).
In summary, 22Cr2Ni4MoV steel is a high-performance alloy known for its excellent hardenability. After forging, the material tends to form a martensitic structure with high hardness and coarse grains. To achieve a more balanced microstructure suitable for subsequent processing, normalization and tempering are recommended. By following appropriate heat treatment procedures, such as normalizing at 920°C and tempering at 680°C, the material can be transformed into a ferrite + pearlite structure, reducing hardness to around 22–24 HRC and improving machinability and formability.
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